Random bred, male Fisher 344 rats, CD-1 mice, and golden Syrian hamsters were exposed to 1-2 ppm of NO2 for 6 hr/day, 5 da/wk for 30 weeks. Some of the animals also received 0.1% morpholine in deionized water as their sole source of drinking water. The animals will be held for 50 wk after the 30 wk NO2 exposure period during which time all animals will receive deionized water. Animals will be killed for histopatological evaluation following the postexposure holding period. Future studies will include other heterocyclic amines, e.g., piperazine, 2,6-dimethylmorpholine.